Colin Walters authored Nov 13, 2012 and Matthias Clasen committed Nov 19, 2012

1) For embedded systems, it makes no sense to include documentation
   of this kind on each image.  So we might as well not even build
   it.
2) For people bootstrapping systems from source code, documentation
   is the source of many cyclical build loops.  Allowing it to
    be disabled helps cut these loops.
3) The Docbook stylesheets and build system have no sane upstream,
   and so at the moment aren't included in gnome-ostree.

https://bugzilla.gnome.org/show_bug.cgi?id=688271